High-strength weldable constructional steel with high manganese



United States- Patent Oflice Int. Cl. C22c 39/00 US. Cl. 75-423 8 Claims ABSTRACT OF THE DISCLOSURE A low-alloy, substantially pearlite-free. steel having high tensile strength and toughness in combination with good welding properties. The steel contains, besides iron, 0.0l-0.04% carbon, 2.2-6% manganese,up to 1% silicon, and up to 0.2% acid-soluble aluminum, together with at least one of the elements Nb, V and Zr in amounts of 0.01-0.2% each, and normal amounts of usual impurities.

The present invention is concerned with a low-alloy steel which is pearlite-free or substantially pearlite-free and is distinguished by its high tensile strength, resistance to fatigue and toughness in hot-rolled air cooled condition, and which moreover has good welding properties.

It is a known fact that the strength properties of weldable ferrite-pearlitic steels are highly dependent on the grain size of these steels; this applies especially to their yield point and to their impact critical temperature. It is possible to reduce their grain size and thus to improve their aforesaid properties by means of using a low finishing temperature in the hot rolling operation and by means of additives which have the efiect of diminishing the grain size and in some cases also of causing or promoting precipitation hardening. Such steels, however, will often show a strongly pearlite-striped configuration which deteriorates their ductility-especially theii capacity of bending in a direction parallel to the rolling directionand their strength in the direction of their thickness.

For this reason, investigations have been concentrated on the properties of pearlite-free steels, that is, steels having a carbon content which is so low that practically no pearlite is formed when the steels are allowed to cool in the air. It was found that in normalized iron-manganese alloys containing up to 5% Mn and about 0.004% C the yield point (in certain cases the 0.5% yield point) rises from 13 kp./sq. mm. at Mn to 23 kp./ sq. mm. at 3% Mn and to 48 kp./ sq. mm. at 5% Mn. The impact critical temperature varies within the range of from -20 C. to --50 C. in steels containing up to 3% Mn and is about +100 C. at 5% Mn. On the other hand, steels containing C in amounts of 0.03-0.05 and Mn in amounts of up to 2% have a much higher impact critical temperature, while their yield point remains practically unchanged.

It is also known (see Swedish patent specification 155,512) that a pearlite-free steel containing less than 0.015% C, about 0.1% P, 0.015% N and 0.20% Mn has a yield point of about 30 kp./sq. mm. and an impact critical temperature of about -25 C. An increase of the manganese content in this steel deteriorates its ductility properties at lower temperatures.

A further fact which is known is that in pearlite-free or almost pearlite-free steels of the type which contain less than 0.1% C (usually 0.04-0.05% C) and up to or less than 2% Mn and which have been subjected to a fine-graining treatment with niobium, niobium-f-nitrogen, vanadium+nitrogen or niobium+vanadium+nitrogen the yield point-especially if the carbon content is very low Patented June 30, 1970 and if the content of manganese is 1.2-2%-will increase with decreasing final rolling temperatures down to 950- 850 C. and the impact critical temperature will decrease with decreasing final rolling temperatures down to about 840 C. As an example, it may be mentioned that a steel containing 0.08% C, 1.4% Mn, 0.03% Nb and 0.008% N and rolled at a finishing temperature of 841 C. has a yield point of 44 kp. sq. mm. and an impact critical temperature of -1l0 C. (15% fibrous fracture).

Moreover, it is known that for securing satisfactory welda-bility it is desirable to avoid too high manganese contents in ordinary steels. It is thus recommended that the carbon equivalent (Percent +Percent; Mn)

should not exceed the value 0.40. This limits the manganese content for such steels to very little above 2%.

The present invention is concerned with a steel having the following analysis:

The present invention is especially concerned with a steel containing more than 2.2% of manganese, it having been found that the aforesaid, empirical limit value for the carbon equivalent does not apply to certain special manganese steels of low carbon content, these steels having been subjected to a fine-graining treatment and having the composition as stated above. The strength of the steels increases with increasing manganese contents when these latter exceed about 2.2%, and at the same time the impact critical temperature remains low and the weldability remains good. This is believed to be due to the effect that as the proportion of manganese is increased the steel will contain an increasing amount of a conversion product formed at low temperature. When the Mn content amounts to 4% the structure consists wholly of this conversion product which is in a very finely divided state. This fineness of the grain structure, in combination with the very low carbon content, appears to 'be responsible for the good toughness properties of the steel.

A major advantageof the steels according to this invention resides in the fact that their strength properties are independent of the finishing temperature, within a relative wide temperature range. The strength properties of the steels according to this invention are controlled mainly by the choice of their manganese contents. Thus for instance the yield point can be varied within a range of from 50 to kp./sq. mm.

It the steel contains 4% of manganese its properties are fully equivalent to, and in some respects (toughness, fatigue strength) superior to those of the tough-hardening steel SIS 2541. Moreover its good 'weldability is an outstanding and surprising feature which makes this steel useful for a much wider field of applications as compared to known steels of similar strength characteristics.

The test data below will further illustrate the invention by way of examples, but it should be noted that the invention is not restricted to these examples. The strength tests were carried out on rods of 12 x 12 mm. cross section which had been hot-worked and then been allowed to cool freely.

3 4 g I COMPOSITION 7 sible and unobjectionable in connection with welding operations. Percent The steels of the present invention have very good Steel No. Si Mn Nb V Al N strength properties in the hot-rolled state. They contain [L017 only manganese as a major alloying component, in addi- 0014 tion to minor proportions of fine-graining elements. The 0:017 steels therefore are less expensive in manufacture than (1010 those alloyed with Ni, Mo etc. which have to be toughhardened in order to attain similar properties.

The steels may be produced by remelting a low-carbon STRENGTH PROPERTIES 10 base material or by oxygen fining in for instance a Martin ,pmpmre, 5,111), furnace, arc furnace, LD furnace or Kaldo furnace. The Percent low carbon content may be obtained also by means of 72 i 25 60 degassing in vacuum. The steels are alloyed with man- 35 i; 23 ganese by means of additions of manganese metal or fer- 33 53 gg romanganese of low carbon content. 104 14 5 We claim:

1. In a low-alloy, pearlite-free steel containing man- IMPACT STRENGTH Charpy with V notch, k.p.m. C .t 1

n ma Steel N0. C. 0 C. 20 C. 40 C. -fi0 C. --75 C. temp, C.

3 Lower than 40 11 Lower than 75. 2.9 -lo.

12 Lower than 75. 7 Lower than 75. 20

The expression critical temperature in the impact testganese, the improvement that the steel contains, ing operation means the temperature at which the impact 0.010.04% carbon strength is-2.8 kp. m. 2.2-6% carbon As pointed out above, the strength of the steels is relaup to 1% silicon tively independent of the finishing or final rolling temperaup to 0.2% acid-soluble aluminum ture. A steel according to the invention, steel No. 7, which 0.01-0.2% of at least one element selected from the contained 0.040% C, 0.41% Si, 4.1% Mn, 0.01% Nb, group consisting of Nb,Vand Zr 0.043% A1 and 0.015% N was rolled to a 12 mm. diamthe balance being iron and normal amounts of usual imeterlat finishing temperatures of between 860 and 770 C.; purities, to thus bestow upon said steel a high tensile the following hardness and grain size values were obstrength and toughness in hot-rolled, air-cooled state, comtained: bined with good welding properties.

, 2. A steel as claimed in claim 1 in which the carbon Finishing temp, o 860 820 810 770 content 15 g n a e s 313 g g g 3. A steel as cla med in claim 1 1n WhlCh the manganese ams mwwns 6 I I Content is 254% 4. A steel as claimed in claim 1 in which the silicon The fatigue strength of steel No. 7 was tested also, with ontent is below 0.5%.

the results as set forth below. By way of comparison the 5. A steel as claimed in claim 1 in which the Sil on fatigue strength of steel SIS 2541 in tough-hardened concontent is O,5 1.0%. I dition was also determined. Sample rods of this steel were 6. A steel as claimed in claim 1 in which the content of taken from a rod which had been rolled to a 15 mm. diamidol bl aluminum is 0.005 0.1%. etel' and then tough-hardened in this dimension. 7. A steel as claimed in claim 1, containing 0.02-0.15% of at least one of said elements selected from the group consisting of Nb, V and Zr.

a-yield ma: (r1 11 pt., kp./ kp./sq. kp./sq. 8. A steel as cla1med1n claim 1, containlng SteelN mm. mm. mm Uf/fl'ppil Ufp/Uppf carbon 7 so ;i;55 405:40 i069 0. 5010.50 3-5% manganese SIS 2541.... 75 its 373:3? $0.64 0. 49:110. 49 up to 1% Silicon zl x=Fatiguia linit (2X106 load alternations) with alternating tensile 0.0l0.l% acid-soluble aluminum m t i gt i iiirifi iiXw load alternations) with pulsating tensile 0'01 O'1% 0? least one element Selected from the or compressive stresses. group consisting of Nb, V and Zr.

The weldability was tested by means of butt welding R f d flats having a thickness of 5.5 mm. with coated electrodes 9 erences l e (OK 4830) No cracks were formed, and hardness meas- UNITED STATES PATENTS urements through the heat-exposed zones showed that no 911 053 5 1933 Brophy major changes in hardness had occurred. Steel No. 3 1 979 594 11 1934 i11 which prior to the welding had a Vickers hardness of 305 2 1 27 2 1952 Muller 75 124 H showed after the welding a hardness increased to 310-320 H adjacent the Weld and a hardness decreased HYLAND BIZOT, Primary Examiner to 285-290 H in regions at a somewhat greater distance from the weld. It may be mentioned that hardness values US. Cl. X.R. of up to about 350 H are Considered to be quite permis- -124 

